We seek to better define the mechanisms regulating active fluid transport in the small intestine. Studies will be conducted on intestinal mucosa from two phylogenetically separate species: a mammal (rabbit) and a teleost (flounder). Morphologically, these species differ in that the former has crypts with immature, dividing cells and the latter doesn't. The principal regulatory factors with which we will be concerned are cyclic AMP, cyclic GMP, hormones, neurotransmitters and ions (H, Na, K, Ca, Cl, HCO3). We will be especially concerned with membrane carriers and permeabilities for anions, including their kinetic properties, structural affinities, dependence on Na and their localization to crypts or villi and to luminal or basolateral cell surfaces. Studies will be conducted with both intact epithelium and isolated epithelial cells.